HMS M1 was a submarine of the British Royal Navy, one of four vessels of her class ordered towards the end of the First World War.

The vessels were originally intended as "submarine monitors", but their purpose had been changed before detailed design began.

"[1] Although the gun had an effective range of 15,000 yards (13,700 m), it was normally fired using a simple bead sight at periscope depth with only the barrel above the water.

[2] She sank with all 69 hands in 70 metres (230 ft) of water on 12 November 1925 while on an exercise in the English Channel when a Swedish ship, SS Vidar, struck her while she was submerged.

Later that year, Richard Larn and a BBC TV documentary crew visited the wreck, and the resulting film was broadcast in March 2000.